NSX: Pass NSX uuid when plugging l2 gw attachment

The code passed the neutron network identifier.
This is no longer valid as identifiers on the backend are not
anymore the same as neutron identifiers.

Also, this would have hardly worked with networks on extended
logical switches.

Closes-Bug: 1285399
Change-Id: I6fe4d22d5b8d8b7dc3eb1cb3f47ad52ac1a2b565
Merge "NSX: Pass NSX uuid when plugging l2 gw attachment"